The presence of a 1.4 x 10 to the 6th power dalton double-stranded RNA containing virus-like particles (the killer plasmid or killer virus) in Saccharomyces cerevisiae enables cells to secrete a protein toxin and be immune to this toxin. We have defined 24 chromosomal genes (called mak1, . . .) needed to replicate or maintain the killer plasmid. Methods for complementation and genetic mapping of these genes have been devised. A nonprotein mitochondrial gene product inhibits killer plasmid replication. This effect is prevented by the mak10 product. One dominant chromosomal mutation bypasses the plasmid's need for two mak genes, while another dominant chromosomal mutation bypasses the plasmid's need for five other mak genes. One chromosomal gene is needed to maintain the killer plasmid, to maintain mitochondrial DNA, and for mitotic cell growth. Chromosomal mutations resulting in increased production of killer toxin (superkillers) have also been observed. BIBLIOGRAPHIC REFERENCES: Leibowitz, M.J., and Wickner, R.B.: A chromosomal gene required for killer plasmid expression, mating, and sporulation in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. U.S.A. 73: 2061-2065, 1976. Wickner, R.B.: Killer of Saccharomyces cerevisiae: a double-stranded ribonucleic acid plasmid. Bacteriol. Rev. 40: 757-773, 1976.